Frequency responsive servosystem



May 22, 1962 N. B. sAuNDERs 3,036,252

I FREQUENCY RESPONSIVE SERVOSYSTEM Original Filed May 25, 1950 2 Sheets-Sheet 1 @may A May 22, 1962 N. B. sAuNDERs FREQUENCY RESPONSIVE sERvosYsTEM 2 sheets-Sheet 2 Original Filed May 25, 1950 ...MHQSMUNYNQ IWIIII .|11

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3,36,Z52 Patented May 22, 1962 3,036,252 FREQUENCY RESPGNSIVE SERVUSYSTEM Norman B. Saunders, Weston, Mass., assignor to Raytheon Company, a corporation of Deiaware Original application May 25, 1950, Ser. No. 164,283. Di-

vided and this application Apr. 17, 1956, Ser. No.

s Claims. (ci. 31a-2s) This `application is a division of application, Serial No. 164,283, led May 25, 1950, by Norman B. Saunders, now Patent No. 2,841,775.

This invention relates to velocity-determining apparatus, and more particularly to a self-balancing frequencyresponsive bridge circuit whereby the difference or Doppler frequency between the transmitted and reflected energy waves may be determined.

It is known that the velocity of a ship traveling through the water may be determined by transmitting sonic waves from the ship through the water in a direction parallel to the direction of the motion of the ship, and receiving waves which are reflected back from discontinuities in the water, such as air bubbles, impurities, and surface conditions, and comparing the frequency of the received waves with the transmitted waves. The difference frequency will vary with the velocity of the ship and, therefore, the velocity of the ship lmay be measured in terms of this frequency.

This invention discloses a particular system whereby transmitted and received waves may be accurately compared to determine the frequency difference therebetween.

Briefly, the apparatus comprises `a frequency-responsive bridge to which the difference frequency is fed and which has a plurality of output signal channels. The output of one of said signal channels becomes zero for a particular or balance frequency, said frequency being determined by the parameters of the bridge.

A second output channel from the bridge produces a signal which bears a phase relation to the tirs-t signal such that the `difference in phase between these two signals is at all times a multiple of 11- radians. In other words, the second signal is always either in phase with the first signal or 180 degrees out lof phase with the first signal. For example, if the incoming frequency is above the balance frequency of the bridge, the second signal is in phase with the first signal, while if the incoming signal is be low the balance frequency of the bridge, the second signal is out of phase with the first signal.

These two signals are fed to a phase-comparison circuit, the output of which is a unidirectional signal whose polarity depends upon the relative phase of the two signals. Specifically, the phase-comparison circuit comprises a duo-triode, the anode of one section being connected to the cathode of the second and vice versa.

The first signal is fed to one of the anode-cathode pairs which are tied together, and the other signal is fed through a squaring lamplifier to the grids of the duo-triode. The `unidirectional signal is developed 'across an impedance connected to the opposite anode-cathode pair from that `to which one input signal is connected. The unidirectional signal output is used to run a servo motor whose direction of rotation depends on the polarity of the unidirectional signal. The servo motor, in turn, is used to vary the circuit parameters of the bridge thereby varying the balance frequency. The result is a self-balancing bridge circuit which automatically balances at the frequency of the incoming signal.

In addition, this invention discloses means whereby corrections may be introduced into the bridge system to compensate for a variety of errors. For example, the

` velocity of sound in water varies with the density and salinity thereof. To compensate for this variation, a correction circuit is utilized which adds a correction signal to the first signal. This circuit comprises a plurality of variable potentiometers which are fed by a signal derived from the input signal to the bridge. The signal developed at the variable tap of one of the potentiometers is then added to the first signal. The variable tap of the potentiometer is moved by a mechanical linkage ganged to the servo motor whose angular rotational position corresponds to the velocity of sound in Water. The velocity of sound in water is determined by measuring the time required to pass a signal between a transmitting and receiving transducer through the water.

Further, this invention discloses the Vintroduction of a correcting signal to compensate for nonlinearities due to variations of the velocity of the ship through the water, these variations being caused, in part, by variations in the density and salinity thereof. This correction is introduced by means of a pair of potentiometers connected in series with said first potentiometer, one at either end thereof with the variable arms thereof being ganged together and each respectively connected to the outer end of its potentiometer. The result is a composite potentiometer having an output which may be Varied by either of two inputs, the first input being used to correct the speed of the ship through the water and the second input being used to correct the velocity of sound in water.

Other and further objects and advantages of this invention will be apparent as the description thereof progresses, reference being had to the accompanying drawings, wherein:

FIG. 1 illustrates a functional flow diagram of a system embodying this invention; and

FIG. 2 illustrates a schematic diagram of one species of a self-balancing bridge which may be utilized in this invention.

Referring now to FIG. l, there is shown an apparatus for measuring the velocity of a ship through water comprising a pair of transducers 10 and 11, transducer 10 being the transmitting transducer and transducer 11 being a. receiving transducer. By way of example, there is shown a source of signals comprising a one megacycle crystal oscillator 12, the output of which is fed to a one megacycle driver 13 which may be of any desired power amplifier. The output of driver 13 is fed through a transducer tuning unit 14 to the transmitting transducer 10. Sonic signals emitted from transducer 10 are rellected by discontinuities in the water, and the reected signals are picked up by receiving transducer 11. The signals are fed from the transducer 11 through an amplier 15 to a mixer amplifier 16. A signal is also fed from the driver 13 to the mixer amplifier 16. The output of mixer amplifier 16 is the Doppler or difference frequency between the signal transmitted by transducer 10 and the reflected signal received by transducer 11. This difference frequency is fed to a frequency meter 17 of the self-balancing bridge type to be described in detail later.

The output of frequency meter 17 is a unidirectional signal which is fed toa servo amplifier 18. 'The output of servo amplifier 18 is fed to a servo motor 19 which may be, for example, ya reversible direct current motor. Motor 19 mechanically drives a gear reduction unit shown diagrammatically at 20. Gear reduction unit 20 has a plurality of outputs, two of which are fed back to the frequency meter 17. One of these two brings the bridge of rthe frequency meter into balance, `and the other introduces a correction into the meter corresponding to the velocity of the ship through the water. A third output from speed reduction unit 2G is fed through a second speed reduction unit 21 to a dial 22 which indicates the ships speed.

Another mechanical input is fed -to the frequency meter 3 37 for producing a correction therein to compensate for changes in the velocity of sound through water. This input may be controlled in any desired manner. For example, an automatic velocity of sound unit may be utilized which constantly measures the velocity of V sound iin water. As shown here, the velocity of sound unit lco'rn'trrises a 1.04 megacycle crystal oscillator 23, the output of which is fed to a mixer 24. An output signal from the one inegacycle crystal oscillator 12 is also fed to the mixer 24 with the result that the output of mixer -24 is a forty kilocycle signal. This output is fed through a forty kilocycle driver unit 25 to an amplifier modulator unit 2o. Also fed to ampliiier modulator unit 26 is an inputfrom the one megacycie driver 13 with the result dat the output of modulator unit 26 is a one rnegacycle signal modulated in amplitude at forty kilocycies. rl'his output is fed to a transmitting transducer 27 which converts the -signals into sonic waves which are transmitted through the water to a receiving transducer 23. The out- 'put of the transducer 28 is fed through a demodulator 29 to a phase meter 3i), the output of demodulator 29 being the* modulation frequency of forty kilocycles.

A signal from the for-ty kilocycle driver 25 is also fed to the phase meter 30, and the output of phase meter 3Q ibecornesa unidirectional signal whose polarity is deter- -yrr'linedwh'/ the relative phase of the two signal inputs thereto.i he unidirectional signal from phase meter 3@ is fieri through a servo amplier 31 to a reversible direct current motor 32 which is connected to a gear reduction unit 33. One output of gear reduction 33 may be connected to a dial 34 for indicating the Velocity of sound `in water, and another output from gear reduction 33V is fed to the frequency meter 17 fto produce the, correction therein for changes in the velocity of sound through water.

It is Vto be clearly understood that othermethods fo for all frequencies. Similarly, the junction between resistor 591 and condenser ou is adjusted to be equal to two-thirds of the potential of the input signal for all frequencies and 180 degrees outof phase therewith. The junction betweenresistors Stand 59 is connected to the grid 61 of a cathode follower o2 which is one component of a signal adding network. The plate 63 of cathode follower 62 is connected to B+, and the cathode 64 is connected to, ground through a load resistor 65.

Another component of the adding network is a second cathode follower 66, the gri-d 67 of which is connected to the junction between resistors 38 and 39 in the bridge circuit 35. The plate of cathode follower 66 is connected to B+, .and the cathode o9 is connected through a loadresistor 70 to ground. Cathode@ is connected through a variable resistor 71, a resistor 72, a resistor 73, and a variable resistor 74, all in series, to the cathode 64 of tube 62. The junctionl between resistors 72 and 73 constitutes the output of the addingV network and is connected through a coupling condenser 75 to the grid 76 of an ampliiier tube 77. Grid 76 is connected' to ground through a grid load resistor 73. The cathode 79 of tube 77 Vis connected to .ground through a cathode bias resistor Si). The vscreen grid 81 is connected Yto the cathode through a bypass condenser S2 and to B+ through a voltage dropping resistor 83. The p1ate84 is connected to B+ through a plate load resistor 85 and through a coupling' condenser 86 to the grid S7 of a second amplifier tube 88. Grid 87 'is connected to ground through a grid load resistor 89. Cathode 9) of tube 88 is 4connected to ground through a cathode bias resistor 91. The screen grid 92 is connected to Vthe cathode through a signal bypass condenser 93 and to B+ through determining the velocity of sound in watercould be used. Y

sistor 33 in series with inductance 37, `a variable resistor i 39 in series with variable resistor 38 and inductance 37, and an inductance 46 in parallel with variable resistor 39. The arms of variable resistors 38 and 39 are mechanically ganged together and driven Iby one of the outputs of gear reduction unit 20. The impedance comprising ele-k ments 37-40 is connected between the signal input and ground. Y r Y The signal input is further connected through a resistor 41 to ground, and through a resistor 42 and condenser 43 in parallel tothe grid 44 of a phase-inverting amplifier tube 45. The cathode 46- of tubei is connected to vground through a biased resistor 47.` The screen. grid 48 is connected to the cathode through a bypass condenser 49 and to B+ through a voltage dropping resistor 50. VVThe plate 51 is connected througha load resistor a voltage dropping resistor 94. The plate 95 of tube 83 is connected to B+ through a plate load resistor 96 and to the grid 97 of a cathode follower tube 9S. The plate -99 of cathode follower-tube 98 is connected to B+, and

the cathode lthercof is connected through a resistor 101 to the cathode 79 of tube 777. thereby providing substantial degenerative feedback through the amplifier sec- Ytion comprising tubes 77, 88 and'98..

The cathode 1110 of Vtube 9,8 is connected through a resistor '102 and condenser'1tl3 in series to the cathode 19:5 01" a duo-triode 105 which Vcomprises a phase-com parison circuit. lThe cathode 104 of'one triode section is connected to the anode 166 of the other triode section and Vto ground through a load resistor 107. Cathode 104 is also connected througlra resistor 10S to the tap of a voltage Vdivider networkV comprising resistors V109 and 110 connected in series Ibetween B+ and ground. The anode 111 of the triodesection having a cathode 104 is connected to the cathodeV 112 of the other triode section and to ground through aY resistor 113.V Anode 111 is also'connected -to ground through a condenser 114 and resistor 115 in series.` .Anode'lll is also connected to ground through a resistor. 116 and condenser 117 in series.,L The junction between resistor 116 and con- Y Tdenser 117 comprises the output of the phase-comparison Y network and is a unidirectional signal whose polarity 52 to B+ and to the grid 53 of :a cathode follower'tube connected through aresistor 53,' aere'sistor 59 and a condenser 6i), all in series to the cathode 56 of tube 5,4.V

Resistor 59 is shunted by a condenserol.

throughthegrids thereofiasl follows.VIV The junction betweenV inductance 37 and resistor38 in the bridge 35 is connectedrto the grid 118 of a cathode, followerf119 Vwhich Vis one-'component of a second `adding circuit. The cathode- Y 120 of -tube 119 is Vconnected tofground through a' cathode g' f7.0y The parameters of the circuits connected'to the amplr-` filer 45 of the cathode follower 54 .arefso chosen Vthat theV 7 load res,istor121, and theV plate 122 thereof is connected to' B+. The junctionbetween resistor 59 yand condenser K Y Y61BA isV connected Ytolthegrid Y1:22 of'a'cathode follower 123 junction between resistors 'and 5'9'has aepotential which is equal to one third of the potential ofV the input signal v making up another component Vof the second adding cir- 'ct'.tit';V fThe, plate 124'of'tube, 123 is lconnec-tedsto B+,

to the bridge 35 and is 180 degrees outvof phase'therewitlrV ,715 andi the"A cathode' T125 thereof'isj connected 'toi'. ground `104 and the anode 105 of the duo-triode section. `to the large degeneration in the amplifying tube, the outthrough a cathode load resistor 126. Cathode 125 is connected through a resistor 127, potentiometer 128 and 'a resistor 129, all in series, to the cathode 120 of tube 119.

The variable arm of potentiometer 128 is connected through a coupling condenser 130 and grid-current limiting resistor 131 to the grid 132 from a squaring amplifier tube 133. The cathode 134 of tube 133 is grounded. The screen grid 135 is connected through a bypass condenser 136 to the cathode and through -a voltage dropping resistor 137 to B+. The plate 138 is connected through a plate load resistor 139 to B+. The junction between resistor 131 and condenser 130 is connected to ground through a grid load resistor 140. The plate 138 is connected through a coupling condenser 141 and gridcurrent limiting resistor 142 to the grid 143 of the triode section having cathode 104 and plate 111. The junction between condenser 141 and resistor 142 is connected to `cathode 104 through a grid load resistor 144. -Plate 138 is connected through a coupling condenser 145 and a grid-current limiting resistor 146 to grid 147 of the triode section containing cathode 112 and anode 106. The junction between resistor 146 and condenser 145 is connected to cathode 112 through a resistor 148.

The operation of this device will now be described, An

input signal is applied across the bridge which may be, for example, a sine wave having a voltage on the order of fifteen volts. This signal is fed through the inverter tube 45 and the cathode follower 54, the circuit parameters associated with these two tubes being designed such that the output is an extremely linear function of the input for all frequencies and inverted in phase with respect to the input.

The signal fed to the adding tube 62 is, therefore, out of phase with the signal fed to the adding tube 66. Therefore, by adding these two signals through the adding section, the difference between one third of the input signal voltage and the voltage appearing across variable resistor 39 is obtained at the output of the adding section. For a particular frequency, and for particular values of the components of the bridge, the output of this adding tube f will be zero. However, for any other frequency, the out- `put will be a finite value and will be amplified through `the amplifier tubes 77 and 8S and the cathode follower 93 thereby applying a sinusoidal signal to the cathode Due put thereof is relatively linear.

Similarly, the adding section comprising tubes 119 and 123 measures the difference voltage between the junction of choke 37 `and resistor 38, and two thirds of the input voltage. The output of this is fed through the squaring amplifier 133 which clips the peaks of the sinusoidal wave form to form a substantial rectangular wave, said rectangular wave then being applied to the grids of the duo- `triode section.

If the rectangular wave drives the grids positive at the time cathode 104 is being driven negative, current will fiow from cathode 104 to anode 111 thereby developing la negative unidirectional output potential across the condenser 117. Similarly, if the grids of the triode section Vare driven-positive when anode 106 is being driven positive, a potential will be developed across condenser 117.

The filter section comprising components 113 through 117 is designed to exactly match the components 102, 103, 108, and 107 at the input side of the triode section. This results in an equalization of the surge impedances for current flow in either direction in the triode section. `In addition, by connecting the grids 143 to 147 back to the cathodes, the current drawn by the grids is fed back to the cathode from which it was drawn to the grid and, therefore, creates no error in the zero balance of the `unidirectional output signal.

The outpu-t signal is then used to drive a servo motor which may be, for example, a reversible direct current `rrrotor through a servo amplifier, said motor varying the resistors 38 and 39 whereby the frequency at which the bridge 35 will produce a zero output from the adding sections 66 and 62 will be varied. Due to the action of the reference voltage developed from the bridge 35 at the output of the adding sections 119 and 123, the polarity of the output of the duo-triode section will be such that the motor always drives the variable resistors 38 and 39 toward the balance condition of the bridge, thus producing a self-balancing circuit.

The connection of anode 106 and cathode 104 to the voltage divider network 109 and 110 .through resistor 108 applies `a very small positive potential, on the order of a fraction of a volt, to anode 106 and cathode 104 which in the absence of a signal input from cathode follower 98 produces a small positive output across condenser 117. The polarization of the servo systems is such that this posiitve signal drives the bridge 35 and ships speed indicator 22 to zero. Thus, if the signal input to bridge 35 remains zero forfan appreciable length of time such as occurs where the ship is not moving or moving at such slow speeds that the Doppler frequency is below 60 cycles corresponding to .l knot to which the system will not respond, drifting of the servo control system to indicate erroneous speeds is prevented by the positive voltage,

In order to add a correction to the system which will compensate for variations in the velocity of sound in water and the velocity of the ship through the water, a component is added to the output signal of the adding sections 62 and 66 in the following manner. The junction between resistor 59 `and condenser 60 which corresponds to two thirds of the input voltage is fed to the grid of a cathode follower tube 151, the plate 152 of which is connected to B+, and the cathode 153 of which is connected through potentiometers 154, and 156, all in series, to ground. The cathode 153 is connected to the movable arm of potentiometer 154 thereby making potentiometer 154 avariable resistor. The cathode 153 is also connected through a resistor 157 to the movable arm of potentiometer 155. The movable arm of potentiometer 156 is connected to ground thereby making potentiometer 156 a variable resistor.

The arm of potentiometer 155 is actuated by a servo motor whose rotational position varies as a function of velocity of sound in the water. The arms of potentiometers 154 and 156 are mechanically ganged together and to the servo motor which balances the bridge 35 thereby introducing a correction corresponding to the input frequency to bridge 35 and which, therefore, is a function of the speed of the ship through the water. The arm of potentiometer 155 is connected through a resistor 158 to the junction between resistors 72 and 73. Thus, it may be seen that a predetermined proportion of the input signal is fed into the adding sections 62 and 66, said portion being controlled first in response to the speed of the ship and second in response to the velocity of sound in the water.

It may be noted that the potentiometer arrangement 154 through 156 coud have substituted therefor a mechanical arrangement embodying a single potentiometer whereby the speed correction could be applied to the movable arm of the potentiometer, and velocity of sound could be applied to the case of the potentiometer thereby moving the resistance card thereof relative to the movable arm. This correction, in effect, varies the frequency which the bridge 35 will balance thereby causing a change in the rotational position of the servo motor with the resultant change in the indication of the sound of the ship through the water.

Since the phase reference signal is fed to the phase comparator only when an input signal is present, the system works accurately even when the input signal is discontinuous and erratic.

As shown here, the signal input to bridge 35 is fed through a resistor 160. Since the impedance of the bridge is lower at low frequencies, a smaller proportion of the over-all input signal is fed to the bridge at low frequencies. This compensates for the characteristic of the bridge system to react quicker at low frequencies,

This completes the description of the particular Vembodiment of the invention described herein. However, many modifications thereof will be apparent to persons skilled in the art without departing from the spirit and scope of this invention. For example, other types of bridges could be used, and the invention is not necessarily limited to `the particular type of phase-comparison triode section described herein.

Furthermore, the system is not limited to the measurement of speed of ships through the water but is also applicable to the use of sound to measure the velocity of objects through the air, and, in addition, could use electromagnetic radiations rather than sonic radiations. Therefore, applicant does not wish to be limited to the par-V ticular details of the invention described herein except as defined by the appended claims.

What is claimed is:

1. A frequency-responsive electron-discharge device comprising a bridge circuit having first and second output terminals and a phase inverter circuit having first and second output terminals; means for applying an input signal to each of said circuits; a first channel means Vincluding a first comparison means having two inputs, one of said inputs connected to said first bridge output terminal and the other of said inputs connected to said first phase inverter output terminal, said first comparison means thereby adding the output signals produced at said rst bridge and said first phase inverter output terminals to derive a first control signal; a second channel means including a second comparison means having two inputs, one of said inputs connected to said second bridge output terminal and the other of said inputs connected to said second 'phase inverter output terminal, Saidv second com-V parison means thereby` adding the output signals producedl at said second bridge and said second phase inverter output terminals to derive a second control'signal the phase of which always differs from the phase ofi said first control signal by an integral multiple of vr radians.

2. A frequency-responsive electron-discharge device comprising a bridge circuit having first and second output terminals and a phase inverter circuit having first and second output terminals; means for applying au input signal to each of said circuits; a first channel means in,

terminals to derive a firstv control signal, said first con-.

trol signal being zero for a balance frequency; a second channel means including a second comparison means having two inputs, one of said inputs connected to said second bridge output terminal and the other of said in- Y puts connected to said second phase inverter output ter'- minal, said second comparison means thereby adding the output signals produced at said second bridge and saidV second phase inverter output terminals to derive a second control signal the phase of whichrdiflers Vby, an even multiple of 1r radians from the phase of saidfirst control signal when the frequency of said input signal is above the balance frequency of said bridge and the phaseV one of said inputs connected to said first bridge output terminal and the other of said; inputs connected to said first phase inverter output terminal, said first comparison means thereby adding the output signals produced at said first bridge and said fir-st phase inverter output terminals to derive a first control signal; a second channel means including a second comparison means having two inputs, one of said inputs connected to said second bridge output terminal and the other of said inputs connected to said second phase inverter output terminal, said second comparison means thereby adding the output signals produced at said second ,bridge and said second phase inverter output terminals to derive a second control signal the phase of which always differs from the phase of saidfirst control Signal by an integral multiple of fr radians; phase comparison means having a first input connected to said first channel means and a second input connected to said second channel means, said phase comparison means being responsive to said first and to said second control signals `for producing a servo input signal, servo means connected toV said phase comparison means and to said bridge circuit and repsonsive to said servo input signal for varying the balance frequency of said bridge circuit.

4. A frequency-responsive electron-discharge device comprising a bridge circuit lhaving first and second output terminals and a phase inverter circuit having first and second output terminals; means for applying an input sign-al to each of said circuits; Ia first channel means including a firs-t comparison Imeans having two inputs, one of said inputs connected to said first bridge output terminal and the other of said inputs connected to said first phase inverter output terminal, said first comparison means thereby adding the output signals produced at said-first ybridgezrmd rsaid first phase inverter output terminals to derive a first control signal; a second channel means including a second comp-arison means having two inputs, one of said inputs connected to said second bridge output termina-l and the other of said inputs connected tosaid second phase inverter output terminal, said sccond comparison means thereby adding the output signals produced at said second bridge and 'said second phase inverter output-terminals' to derive a `second control signal Ithe phase of which lalways ydiffers VfromV the phase of said first signal by lan integral multiple of 1r radians; phase comparison means having a first input connected to said first channel means and a second input connected Y to said second channel means, saidphase comparison means being responsive to said first and lto said second control signals for producing a unidirectional servo input signal the polarity of which `depends on theY relative phases of said first and said second control signals; and servo means connectedV to said phase comparison means yand to said bridge circuit means Yand responsive Vto said servo input signal for varying the balance frequency of said bridge-circuit in a direction dependent upon said polarity.

of which differs by an odd multiple of 1r radians from the phase of said first control signal when therfrequency of said input signal is below the balance frequency of said bridge. K i

3."A frequency-responsive electron-discharge Ydevice comprising a bridge circuit having first and second out`.;

put terminals and a phase inverter circuit having first .and Second output terminalsg-meansrfor applying an input signal to each of said circuits; a first channel means including a first comparison means. having two4 inputs,

5. A 4frequency-responsive electron-discharge' device comprising a vbridge circuit having firstV and second output terminals and a phase inverter circuit having first and second output terminals; means forapply'ing an input signal to-each-of said circuits; a first channel means including a first comparison means having two inputs, one of said inputs connected tol said first bridge output terminal and lthe other of said inputs'connected to said first phase inverter output terminal, said first comparison means therebywaddingfthe output sign-als produced-at said first bridge and said'Y first phase VVinverter output terminals to 1deriveyafrst control signal; `a second channel means including a second comparison meanshaving two inputs, one of saidV inputs connected to said second bridge outputjterminal fand theother of said inputs connected to said second phase inverterV output terminal, said sec- 9 inverter `output terminals to derive -a second control signal the phase of which always ldiffers from the phase of said rst signal by :an integral multiple of 1r radians; phase comparison means comprising ia pair of electrondischarge devices, each of said devices having a cathode, an anode, and -a grid; means for coupling said first control sign-al to the anode of one of said devices and to the cathode of the other of said devices; means for coupling the lgrids of said devices to said second control signal, said phase comparison means thereby producing `a. unidirectional servo input signal `at the junction of the anode of said other device yand the cathode of said one device; servo means connected to said junction `and to said 10 bridge circuit and `responsive to said servo input signal for varying the ibalance frequency of said bridge circuit.

References Cited in the le of this patent UNITED STATES PATENTS 2,146,526 Buschbeck Feb. 7, 1939 2,290,327 Hansel'l July 21, 1942 2,480,128 Frum Aug. 30, 1949 v2,489,262 Buckbee Nov. 29, 1949 2,559,680 Seeker July 10, 1951 2,708,718 Weiss May 17, 1955 

